feat: engine-specific TrimAxis and rename ShrinkAxis.Height to Length

Make quantity trimming direction-aware: DefaultNestEngine uses TrimAxis
(virtual property on NestEngineBase) so HorizontalRemnantEngine removes
topmost parts instead of rightmost. Rename ShrinkAxis.Height → Length
for consistency with Width/Length naming used throughout the codebase.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

OpenNest.Engine/DefaultNestEngine.cs

@@ -64,7 +64,7 @@ namespace OpenNest
             var best = context.CurrentBest ?? new List<Part>();
 
             if (item.Quantity > 0 && best.Count > item.Quantity)
-                best = ShrinkFiller.TrimToCount(best, item.Quantity, ShrinkAxis.Width);
+                best = ShrinkFiller.TrimToCount(best, item.Quantity, TrimAxis);
 
             ReportProgress(progress, new ProgressReport
             {

OpenNest.Engine/Fill/IterativeShrinkFiller.cs

@@ -18,7 +18,7 @@ namespace OpenNest.Engine.Fill
     /// <summary>
     /// Composes <see cref="RemnantFiller"/> and <see cref="ShrinkFiller"/> with
     /// dual-direction shrink selection. Wraps the caller's fill function in a
-    /// closure that tries both <see cref="ShrinkAxis.Height"/> and
+    /// closure that tries both <see cref="ShrinkAxis.Length"/> and
     /// <see cref="ShrinkAxis.Width"/>, picks the better <see cref="FillScore"/>,
     /// and passes the wrapper to <see cref="RemnantFiller.FillItems"/>.
     /// </summary>
@@ -85,7 +85,7 @@ namespace OpenNest.Engine.Fill
                 ShrinkResult widthResult = null;
 
                 Parallel.Invoke(
-                    () => heightResult = ShrinkFiller.Shrink(fillFunc, ni, box, spacing, ShrinkAxis.Height, token,
+                    () => heightResult = ShrinkFiller.Shrink(fillFunc, ni, box, spacing, ShrinkAxis.Length, token,
                         targetCount: target, progress: progress, plateNumber: plateNumber, placedParts: placedSoFar),
                     () => widthResult = ShrinkFiller.Shrink(wFillFunc, ni, box, spacing, ShrinkAxis.Width, token,
                         targetCount: target, progress: progress, plateNumber: plateNumber, placedParts: placedSoFar)

OpenNest.Engine/Fill/ShrinkFiller.cs

@@ -7,7 +7,7 @@ using System.Threading;
 
 namespace OpenNest.Engine.Fill
 {
-    public enum ShrinkAxis { Width, Height }
+    public enum ShrinkAxis { Width, Length }
 
     public class ShrinkResult
     {
@@ -101,7 +101,7 @@ namespace OpenNest.Engine.Fill
             if (bbox.Width <= 0 || bbox.Length <= 0)
                 return box;
 
-            var maxDim = axis == ShrinkAxis.Height ? box.Length : box.Width;
+            var maxDim = axis == ShrinkAxis.Length ? box.Length : box.Width;
 
             // Use FillBestFit for a fast, accurate rectangle count on the full box.
             var bin = new Bin { Size = new Size(box.Width, box.Length) };
@@ -121,7 +121,7 @@ namespace OpenNest.Engine.Fill
             if (estimate <= 0 || estimate >= maxDim)
                 return box;
 
-            return axis == ShrinkAxis.Height
+            return axis == ShrinkAxis.Length
                 ? new Box(box.X, box.Y, box.Width, estimate)
                 : new Box(box.X, box.Y, estimate, box.Length);
         }

OpenNest.Engine/HorizontalRemnantEngine.cs

@@ -24,6 +24,8 @@ namespace OpenNest
 
         public override NestDirection? PreferredDirection => NestDirection.Vertical;
 
+        public override ShrinkAxis TrimAxis => ShrinkAxis.Length;
+
         public override List<double> BuildAngles(NestItem item, double bestRotation, Box workArea)
         {
             var baseAngles = new List<double> { bestRotation, bestRotation + Angle.HalfPI };

OpenNest.Engine/NestEngineBase.cs

@@ -43,6 +43,8 @@ namespace OpenNest
 
         public virtual NestDirection? PreferredDirection => null;
 
+        public virtual ShrinkAxis TrimAxis => ShrinkAxis.Width;
+
         public virtual List<double> BuildAngles(NestItem item, double bestRotation, Box workArea)
         {
             return new List<double> { bestRotation, bestRotation + OpenNest.Math.Angle.HalfPI };

OpenNest.Tests/ShrinkFillerTests.cs

@@ -30,7 +30,7 @@ public class ShrinkFillerTests
             return engine.Fill(ni, b, null, System.Threading.CancellationToken.None);
         };
 
-        var result = ShrinkFiller.Shrink(fillFunc, item, box, 1.0, ShrinkAxis.Height);
+        var result = ShrinkFiller.Shrink(fillFunc, item, box, 1.0, ShrinkAxis.Length);
 
         Assert.NotNull(result);
         Assert.True(result.Parts.Count > 0);
@@ -73,7 +73,7 @@ public class ShrinkFillerTests
             new List<Part> { TestHelpers.MakePartAt(0, 0, 10) };
 
         var result = ShrinkFiller.Shrink(fillFunc, item, box, 1.0,
-            ShrinkAxis.Height, token: cts.Token);
+            ShrinkAxis.Length, token: cts.Token);
 
         Assert.NotNull(result);
         Assert.True(result.Parts.Count > 0);
@@ -97,7 +97,7 @@ public class ShrinkFillerTests
     }
 
     [Fact]
-    public void TrimToCount_Height_KeepsPartsNearestToOrigin()
+    public void TrimToCount_Length_KeepsPartsNearestToOrigin()
     {
         var parts = new List<Part>
         {
@@ -107,7 +107,7 @@ public class ShrinkFillerTests
             TestHelpers.MakePartAt(0, 30, 5),  // Top = 35
         };
 
-        var trimmed = ShrinkFiller.TrimToCount(parts, 2, ShrinkAxis.Height);
+        var trimmed = ShrinkFiller.TrimToCount(parts, 2, ShrinkAxis.Length);
 
         Assert.Equal(2, trimmed.Count);
         Assert.True(trimmed.All(p => p.BoundingBox.Top <= 15));